1. Field of the Invention
The present invention relates to a GaN-crystal free-standing substrate suitable for fabrication of a light-emitting device having high color purity; and a method for producing the GaN-crystal free-standing substrate.
2. Description of the Related Art
Substrates used for light-emitting devices are required to have individual characteristics depending on the types of light-emitting devices. For example, Japanese Unexamined Patent Application Publication No. 2000-049374 discloses that, to provide white-light emission by deliberately using light emission (hereafter, referred to as yellow-light emission) having a broad peak wavelength range including 550 nm and wavelengths around 550 nm from a substrate, a substrate that provides high-intensity yellow-light emission is used. The presence of yellow-light emission in light-emitting devices that provide monocolor light emission degrades the color purity of the light emission. To suppress such degradation, Japanese Unexamined Patent Application Publication No. 11-1396 discloses a GaN substrate having been made to have a high purity. Japanese Unexamined Patent Application Publication No. 2007-126320 discloses a GaN substrate having a low absorptivity and electrical conductivity.